Direct alcoholic fuel cells convert an alcohol fuel to hydrogen ions and carbon dioxide internally through a reaction analogous to the following for a direct methanol fuel cell:Anode: CH3OH+H2O=>CO2+6H++6e−  (1)Hydrogen ions are conducted from the anode through the polymer electrolyte. Electrons are conducted to the cathode through the external electric circuit where oxygen is reduced to water according to the reaction:Cathode: 3/2O2+6H++6e−=>3H2O  (2)The complete electrochemical reaction is:CH3OH+3/2O2+H2O=>CO2+3H2O  (3)As shown by this reaction, the fuel cell relies on the presence of liquid water to efficiently conduct the protons. The theoretical voltage of the fuel cell based upon this chemistry is 1.18 V.
The hydrogen ion conducting membrane separating the anode and the cathode of the fuel cell is an acidic proton conducting polymer selected from commercially available solid polymer electrolytes such as NAFION (a sulfonated tetrafluorethylene copolymer; DuPont Fuel Cells, Fayetteville, N.C., USA), Flemion (a perfluorinated carboxylic acid membrane), and ACIPLEX-S (perfluorosulfonated ionomer membrane, Asahi Chemical Company, Kawasaki, Japan).
Maintenance of fuel cell efficiency depends on the ability to retain the methanol solution in the methanol fuel compartment on the anode side of the membrane, adequate fuel replenishment and on a low internal resistance in the cell.
Methanol diffusion through the electrolyte membrane causes a phenomena known as “chemical short circuiting” of the fuel cell. Methanol which reaches the cathode reacts wastefully with oxygen in a similar fashion to reaction (3). However, in this wasteful reaction electrons do not traverse the external electrical circuit and cannot provide useful electrical energy. This situation is aggravated when methanol concentrations in the anode fuel compartment are raised because a high methanol concentration is a driving force for diffusion of methanol through the membrane.
On the other hand, maintenance of the electrochemical reaction rates depends on the supply of adequate methanol. Denudation of the methanol concentration leads to reduced power generation.
In a DMFC stack, the fuel is circulated through the stack and deplenished fuel is returned to the fuel compartment. Since part of the methanol is used by the electrochemical reaction, the methanol concentration in the compartment is reduced. Consequently, the individual cell and whole stack impedance will change unless the methanol concentration is maintained. The methanol concentration increases when water is lost thereby resulting in impedance increase.
It is therefore desirable to control methanol concentration in fuel cells in order to optimize efficiency of the DMFC stack and maintain output.
Satisfactory control can be achieved by measuring the methanol concentration and compensating for methanol consumption. Consumption of fuel can be calculated on the basis of the electrical charge transferred. The methanol concentration can be maintained at a specified level by addition of water as diluent or addition of alcohol as a concentrate or as a pure substance.
Water and methanol can evaporate from the fuel tank, thereby affecting the methanol concentration. In addition, methanol is lost by diffusion through the polymer electrolyte membrane (PEM) to the cathode side. These concentration changes can be significant and may cause large deviations from the ideal alcohol concentration.
Conventionally, methanol concentration changes caused by the other mechanisms are detected by intrusive sampling and measurement techniques based on various principles such as refractive index of the liquid, the speed of sound in the liquid and the liquid density. A sensor may conveniently be placed, for example, in the fuel reservoir or in the channels connecting the fuel reservoir and the fuel cell stack. The properties measured depend on the composition of the liquid. However, the properties measured are affected by the presence of CO2 bubbles in the liquid and the signal is prone to inaccuracies caused by gaseous inclusions yielding unreliable measurements.
A general teaching of electrochemical impedance spectroscopy (EIS) is provided by Gamry Instruments (Warminster, Pa., USA) at gamry with the extension .com/App Notes/EIS Primer/EIS Primer 2007.pdf of the world wide web.
EP1820040 discloses methods for detecting and indicating faulty conditions in electrochemical cells.
U.S. Pat. No. 7,201,980 discloses a fuel cell apparatus and method for feeding fuel to a fuel cell wherein information on the methanol concentration detected by a concentration sensor is sent to a controller and referred to when the fuel mixer adjusts the methanol concentration of the mixed solution. The concentration sensor provided immediately before the fuel cell is disclosed to achieve power generation while detecting the substantial methanol concentration of the mixed solution fed to the fuel cell.
U.S. Pat. No. 6,824,899 discloses apparatus and methods for regulating methanol concentration in a direct methanol fuel cell system without the need for a methanol concentration sensor. One or more operating characteristics of the fuel cell, such as the potential across the load, open circuit potential, potential at the anode proximate to the end of the fuel flow path and short circuit current of the fuel cell, are used to actively control the methanol concentration.
U.S. Pat. No. 6,794,067 discloses a direct oxidation fuel cell system in which the source fuel is diluted with a diluting fluid prior to entering the fuel cell. For a DMFC in which the methanol source fuel is diluted with water, the dielectric constant of the fuel mix comprising the source fuel and the diluting fluid is measured to determine the relative proportions of source fuel and diluting fluid within this fuel mix. This measurement is then used in a feedback loop to control the subsequent mixing of the source fuel with the diluting fluid, and in particular, to adjust the mix in the event the fuel mix is too rich or too dilute as compared to a desired mixing proportion. Additionally, a second dielectric constant measurement is used to determine the source fuel level of a fuel tank providing source fuel to the fuel cell. An optional telecommunications link is used to automatically order a source fuel refill when the source fuel runs low.
U.S. Pat. No. 6,698,278 discloses a method for indirect measurement of fuel concentration in a liquid feed fuel cell wherein fuel concentration in the fuel stream is calculated as a function of the observed current, the temperature of the fuel stream entering the fuel cell stack, and the temperature of the fuel cell stack itself, thereby eliminating the need for a separate sensor.